For the last few years everytime I've decided to look at implementing a CMS I've got stuck.  It's not because there isn't a good CMS out there - but that I've yet to find one that scratches all my itches.

I'm about to setup a new website, which will require multiple end-users to update the site, each with a varying level of experience.  Ideally I'd like to pick a 'popular' CMS, so that if they get into difficulty there's probably someone else that has also got into difficulty with the same problem.  I'd also like to be able to take advantage of extra content-creating plugins that the end users may find useful.

However, I also want the ability to add a few 'custom touches' - such as easy to modify CSS, widget-able features (such as a couple of twitter streams that'll appear only on certain pages).

I also want to run multiple sites which both share the same theme, and user database - but have different content.

If anyone can make a suggestion as to what I should look into, I'd be more than happy with some feedback.